Sister Tabitha is all about the rules. Rules, rules, rules make her happy; rules, rules, rules give her power and stability. She is, after all, the religious and cultural leader of the village, which means rules and stability are kind of her thing. There's no better way to bring about stability after all, then to enforce rules.
Here's the really creepy thing about Sister Tabitha, though: she wants to be nice. Why is that creepy you ask? Because nobody knows what lengths she's actually willing to go to maintain order. Like the Sisterhood in general, Sister Tabitha seems to have some secrets bubbling beneath the surface. The reason no one knows just how far Sister T will go to keep order, of course, is because folks generally follow the rules… when they don't, they get gobbled by a flesh-eating zombie, so it's kind of a no-brainer (ha ha… get it?).
It's people like our girl Mary that bring out the Trunchbull in Sister Tabitha. Mary isn't so sure she wants to follow the rules after all, and she's itching to find a way out of the village, which pretty much makes her rebel number one in Sister T's eyes.
Mary's a threat because as far as Sister T is concerned, the village is the only safe place for humanity (regardless of whether there are other villages) and the rules that are in place help keep it this way. From Sister T's perspective, Mary—with her curiosity and questioning—might as well just tear down the fence and feed the villagers one by one to the zombies. If that happens, everything Sister Tabitha has worked for will amount to one epic failure… nobody wants that to happen.
Plus, Sister Tabitha is a religious zealot who believes that the zombies are a curse from God, sent because regular folks were "trying to cheat God. Trying to cheat death. Trying to change His will. […]This is His retribution. This is our penance" (8.9). So this whole rebellious escape idea? To Sister T, it's like attacking Iron Man with a kitchen knife. It won't work, and will only end up destroying them all.
Which Sister T is not gonna let happen, so she uses mind games and scare tactics to get Mary under control, watching her like a hawk and up and feeding Gabrielle to the zombies. All in the name of God and security.
It's hard to say whether Sister Tabitha was right or wrong in the end. She dies, thanks to Gabrielle's zombie revenge on the whole town; she also dies though, thanks to trying to hold the Cathedral door open long enough for Mary to enter it.
Was her death her punishment from God? And if so, for what? Killing Gabrielle, or trying to save Mary, the other rebel? Were Sister Tabitha's rules and tactics really selfless, or were they cruel and unusual?
Questions abound when Sister Tabitha bites the dust.